The present invention concerns titanium diboride/boron carbide composite powders, a process for preparing these powders, and densified ceramic compositions comprising titanium diboride and boron carbide.
One result of the study of composite materials has been the characterization of some of these as possessing properties which are significantly improved over the properties of the individual constituents. Ceramic composites in particular, including both fiber-reinforced and multicomponent structures, have been targeted as applicable to a variety of scientific and technological uses. Some of these uses include tooling applications, indenters, nozzles, and so forth. For these and other uses the desirable material should be as lightweight and as tough as possible; however, the attainment of one of these properties has often been accomplished at the expense of the other property.
Boron carbide has been found to exhibit excellent hardness and a relatively low specific gravity, but it lacks toughness (K.sub.Ic =3.6 MN/m.sup.3/2). Titanium diboride, on the other hand, is nearly as hard and much tougher when compared with boron carbide, but it is also much heavier. Because of the potentially complementary properties of these two materials, researchers have directed attention to producing composites comprising both compounds. Results of this research indicate that a ceramic produced therefrom approximates titanium diboride's toughness and exceeds boron carbide's hardness while maintaining a low specific gravity.
Research specifically directed toward titanium diboride/boron carbide composites has yielded, e.g., a 1952 patent, U.S. Pat. No. 2,613,154, which discloses the manufacture of titanium diboride/boron carbide composites from a mixture of boron-rich boron carbide and titanium powders. This method does not, however, appear to be suitable for producing a variety of titanium diboride/boron carbide compositions without the incorporation of excess carbon or boron in the densified piece. The same problem is encountered in connection with research done by Russian workers, as disclosed in E. V. Marek, "Reaction of the boron carbide with Group IV transition metals of the Periodic Table," Mater. Izdeliya, Poluchaemye Metodom Poroshk. Metall., Dokl. Nauchn. Konf. Aspir. Molodykh Issled. Inst. Probl. Materialoved. Akad. Nauk Ukr. SSR, 6th, 7th, Meeting Date 1972-1973, 156-9. This paper describes mixtures of boron, carbon and titanium which are hot-pressed to composites comprising B.sub.4 C and TiB.sub.2 phases. A microhardness superior to that of either B.sub.4 C or the borides is reported.
The Japanese literature, notably Japanese Patent Application 1985-235764, discloses boron carbide/titanium diboride composites prepared by dispersing boron carbide powder and titanium diboride powder in organic solvents such as toluene, and ball milling using a tungsten carbide-cobalt alloy as a milling medium. This material is then dried and cold-pressed. The authors report a hardness approaching that of diamond for a sintered piece prepared from 40 to 50 percent titanium diboride.
Japanese workers also disclose, in U.S. Pat. No. 4,029,000, a boron carbide/titanium diboride composite, prepared from a physical mixture of powders, for use as an injection pump for molten metals. The particle diameter is in the range of 2 to 6 .mu.m for the boron carbide and 5 to 15 .mu.m for the titanium diboride. The hardness attained upon sintering is reported to be lower than that of boron carbide alone.
Research has also been directed toward other composites comprising titanium, boron and carbon. For example, the literature also describes various methods of preparing composite materials comprising titanium carbide and titanium borides. Among these are, e.g., U.S. Pat. Nos. 4,138,456 and 3,804,034, which describes preparation of a TiC/TiB.sub.2 composite and a TiC/TiB/B.sub.4 C composite, respectively, produced from physical mixtures of powders. U.S. Pat. No. 4,266,977 discloses preparation of a composite prepared in a plasma reactor from an "intimate" mixture of the three constituents.
An important parameter in the ultimate utility of a ceramic composite is the degree to which the constituents are dispersed. To realize the maximum benefit of a particulate composite, the components must be uniformly distributed on a microscopic scale. However, such uniform distribution is at best extremely difficult to attain in physical mixtures, such as those produced using any of various milling techniques, in part because of agglomeration of component particles.
A further consideration in producing an "ideal" composite material relates to particle size. This is because the high incidence of failure in engineered ceramic parts can often be attributed to small cracks or voids, which result from incomplete packing of the precursor powders. An obvious solution to this problem would be to use extremely fine composite powders that are substantially uniform as to particle diameter. Such powders would pack more tightly and thereby reduce the number of void spaces formed in the ceramic body. It has been suggested by E. A. Barringer and H. K. Bowen, in "Formation, Packing and Sintering of Monodispersed TiO.sub.2 Powders," J. Amer. Ceram. Soc. 65, C-199 (1982), that an "ideal" ceramic powder for producing a high quality part would be of high purity and contain particles which are monodispersed, i.e., substantially uniform as to size, and which are spherical, nonagglomerated, and fine in size (e.g., less than 1.0 .mu.m).
As a ceramic powder is sintered, adjacent particles fuse into grains. In general, the grain size is governed by the particle size of the powder from which the part is prepared. In other words, the grain size is necessarily larger than the crystalites from which a part is sintered. Thus, the sintering of finer particles presents the opportunity to produce fine-grained bodies. This is especially important in TiB.sub.2 /B.sub.4 C composites, in which the TiB.sub.2 and B.sub.4 C grain sizes should necessarily be less than or equal to about 10 microns in order to maximize the hardness and toughness of the composite. Thus, the particle sizes should be significantly smaller than 10 microns.
The effect of grain size on the integrity of boron carbide bodies having no titanium diboride constituent has been investigated by A. D. Osipov, I. T. Ostapenko, V. V. Slezov, R. V. Tarasov, V. P. Podtykan and N. F. Kartsev, "Effect of Porosity and Grain Size on the Mechanical Properties of Hot-Pressed Boron Carbide," Sov. Powder Metall. Met. Ceram. (Engl. Transl.) 21(1), 55-8 (1982). The authors found that parts exhibiting a fine grain size were significantly stronger than parts consisting of coarse grains.
An additional advantage in the use of ceramic powders with a small average particle size is that the temperatures required to sinter the powders are often reduced. For example, in their work on sintering titanium oxide powders Barringer and Bowen found that the sintering temperature could be reduced from a normal 1,300.degree. C. to 1,400.degree. C. range down to about 800.degree. C. when using 0.08 micron-sized particles. On an industrial scale, this could result in a considerable saving in both material and energy costs.
One method of producing fine ceramic precursor powders is via gas-phase synthesis using a carbon dioxide laser. This method was first developed by Haggerty and coworkers. In the article, "Synthesis and Characteristics of Ceramic Powders Made From Laser-Heated Gases," Ceram. Eng. Sci. Proc. 3, 31 (1982), R. A. Marra and J. S. Haggerty describe the preparation of silicon, silicon carbide and silicon nitride powder from silicon hydride. The powders produced were fine, equiaxed and monodispersed with particle sizes in the range of 100 A to 1,000 A. Their paper also contains the statement that this laser-heated process can be used to produce other nonoxide ceramics such as titanium diboride, aluminum nitride and boron carbide, as well as many oxide ceramics.
However, in that article there is no specific teaching regarding the actual production of boron carbide using a laser. Later work by J. D. Casey and J. S. Haggerty, entitled "Laser-induced vapour phase synthesis of boron and titanium diboride powders," J. Mat. Sci. 22 (1987) 737-744, indicated that the CO.sub.2 laser irradiation of a gaseous mixture of boron trichloride, hydrogen and titanium tetrachloride did not yield any titanium diboride. In sum, none of the described laser pyrolyses appear to have produced a titanium diboride/boron carbide composite powder of any kind in a single step, and certainly not one possessing the superior attributes and unique microstructure of the present invention.